Two closely related polypeptides termed thymin I and II have been isolated from bovine thymus. These 7000 MW polypeptides were shown by immunofluorescence to be secreted exclusively by epithelial cells of the thymus and were not detectable in other tissues. Thymin, in subnanogram concentrations, induced the formation of theta and TL antigens in mouse bone marrow cells cultured in vitro. This action at concentrations which are physiological for most polypeptide hormones suggests that thymin is a differentiation inducing hormone of the thymus and it is possible that thymin I and thymin II may induce sequential differentiation steps. Our studies will be directed towards analysis of the differentiation process induced by thymin and will include studies of the effects of specific metabolic inhibitors on the formation of differentiation antigens as well as studies on the functional capacities of thymin treated cells. These will include the development of mitogen responsiveness, syngeneic and allogeneic mixed leucocyte culture responsiveness, the development of capacity of treated cells to react in a graft versus host reaction and to participate in cell cooperativity experiments. Finally, the possibility of sequential differentiation steps driven by thymin I and thymin II will be explored.